One of the most exciting, and rapidly developing areas within rehabilitation science is the development of advanced prosthetics. In many areas prosthetic limbs are advancing rapidly. New rigid yet lightweight materials such as carbon fiber are being borrowed from the aerospace industry to create high performance feet and legs for amputees (there are currently about 1.6 million amputees in the USA and 3.6 million projected by 2050 wishing to return to an active lifestyle which includes running. At the same time microprocessors and miniaturized electronics are being borrowed from the field of robotics to create artificial joints capable of automatically adapting to a user's walking style and speed.
Despite these advances one of the most critical aspects of prosthetics has yet to be similarly revolutionized. The union of the artificial limb to the soft tissue of the residual limb is known as the soft tissue interface of the prosthetic limb. Currently the most prevalent method of interfacing the prosthetic and the residual limb is by using what is known as a prosthetic socket liner. The prosthetic socket liner is a tight fitting 3-9 mm thick sleeve placed over the limb. It is made of a stretchy polymer such as silicone, polyurethane, or thermoplastic elastomer. These materials are selected because they are relatively inert, they are able to stretch to conform to the shape of the limb, and cushion the limb against the hard surface of the prosthetic socket which is typically made of laminated plastic or carbon fiber. Another role of the liner besides cushioning is to provide effective linkage of the prosthetic to the limb. In order to provide this linkage the liner must be air tight. No existing liners are moisture permeable.
A recurring and persistent problem with current liners is the problem of accumulation of excess moisture in the prosthetic limb. Thus it would be desirable to develop a laminar composite material to enhance the function of existing prosthetic liners by allowing for moisture to pass through it while maintaining an air tight seal.
It would also be desirable to create a socket liner which allows sweat to steadily pass through it for removal from the socket of the prosthetic limb. In a preferred embodiment, one or more parts of the liner will comprise water permeable and/or absorbable materials, which can be cleaned and replaced, or else would allow the user to wear the prosthesis continually without removing it.
A preferred enabling technology of the present disclosure will be the use of an advanced biologically compatible tough hydro-gel composite material which interfaces with the soft tissue of the residual limb.
Major findings from: “Hygiene Problems of Residual Limb and Silicone Liners in Transtibial Amputees Wearing the Total Surface Bearing Socket, 2001”: Forty-seven percent of the subjects complained of excessive perspiration—the level of perspiration being directly related to the number of hours of TSB use, Although an antiperspirant may be temporarily effective, materials for the socket and liner that allow ventilation of the air and a means to radiate heat from the inside of the silicone liner need to be developed.
Eruption and itching on the residual limb, respectively, maybe caused by perspiration and exfoliated outer layers of skin, powder, and deodorant residues, or allergic reactions induced by the materials used in the silicone liner. The odor from the silicone liner, which was noted in 43.4% of the subjects, may also derive from perspiration and dirt.
Excessive heat and moisture retention within the socket are common complaints of lower limb amputees.
The environment between the liner and skin is perfect for forming a host of residual limb skin problems including contact dermatitis, hyperhydrosis, and bacterial infections.
The prevalence of skin problems of the stump in lower limb amputees was estimated as 36%. Skin problems result in a reduction in walking distance without a break and a reduction in prosthesis use.
Moisture and Temperature have been Implicated in Skin Issues:
Hyperhidrosis
Because of the inability of sweat to evaporate (decrease of transport of sweat) and the increased production of sweat because of the cooling reflex of the skin, this isolation will lead to stasis of sweat in the isolated area. As a consequence, hyperhidrosis (an unbalance between production and evacuation of sweat) will occur. Hyperhidrosis can worsen skin problems of the stump, or can be an initiating or supporting factor in the development of skin problems on amputation stumps as mentioned earlier.
Infections
Infections of the skin that occur on other parts of the body can also occur on the stump, such as folliculitis and furuncles. Most types of prostheses involve prolonged contact with the stump, or at least the distal part of the stump, thereby increasing the humidity of the stump socket environment and making it an excellent culture medium for microorganisms such as bacteria, yeasts and mycoses Infections of the skin of the stump caused by microorganisms are therefore common, but are seldom described in detail in the literature as case reports.
Allergic Contact Dermatitis
Friction, sustained pressure and humidity of the amputation stump may not only act as cofactors to increase the chance of allergic contact dermatitis but may also be primary factors in causing irritant contact dermatitis.
Several known approaches for improving prosthetic liners include Ottobock which has a series of antimicrobial polymer liners which are bioactive. They use silver to reduce microbial activity that extends beyond the simple bioinert standard for prosthetic liners. They call it Silvershield®. It uses a measured slow release of ions from the socket into the liner environment to counter act microorganisms.
The Unity system by Charles King is a mechanical device designed to remove moisture via excess pumps, tubes and solenoids. This sort of solution introduces many more points of failure into the device. It makes the device heavier and requires re-training of care givers in the design and manufacture of the product and retraining of end users in its use.
A bioengineering approach to solving problems associated with prosthetic liners are preferred because there is already a system you can put the product into. Active cooling or active moisture reducing systems are not preferred because they are bulky and would take up valuable space and weight away from the overall prosthetic design space. Microprocessor knee joints and ankles will take precedence over active cooling systems. Addition of bulky hardware increases the unique part count and possibility of user error. In addition it adds to the overall weight, and may require a redesign of existing pylons or knee or ankle joints. So it's preferable to design a liner material which can solve many of the problems already existing without the need for developing a new system support paradigm as would be needed in the case of bulky active cooling systems.
By altering the material properties of the liner, it is possible to deliver the same effects of a larger bulkier system with a minimal increase in size and weight of the existing prosthesis. The material science research around prosthetic liners has centered largely on the mechanical properties of the liners. Liner performance is not judged by the mechanical properties of the liners alone however. Aside from cushioning the limb against pressure and shear, the liner also has to have the role of providing a safe micro environment as well. The micro environment, defined here to be the combination of temperature, moisture and ions, plays an important role in creating a biocompatible and successful prosthesis as much as the macro scale mechanical properties as can be seen by examining the rates of patient reported skin problems and satisfaction.
There are a number of technologies which claim to reduce moisture and temperature for prosthetic liners and the like.
Bonded composites have been used in socket liners. These composites are mostly cloth materials bonded to the polymer liners. The cloth provides structural resilience to the liners.
Macro Scale Polymer Suspension Composites
More complex polymer composites including suspensions have also been proposed, such as a liner comprising a suspension of cork granules in silicone as disclosed in U.S. Patent Publ. No, US20120110713. The idea is that the composite material will feature advanced material properties due to the two materials which make it up. This composite is supposed to act as a means of reducing temperature. However, cork is filled with air, which makes it an excellent insulator of heat. Thus, this will not likely work as intended to reduce excess temperature in the socket of the prosthesis.
Micro Scale Polymer Suspension Composites
Ossur has also made a composite liner material comprising air filled microspheres that are imbedded within the silicone liner material which is itself bonded to a nylon fabric. Microspheres introduce air into the polymer, altering its properties. Ossur does not disclose the specific use of the microspheres which may be for thermal or mechanical properties. The focus of Ossur is to protect the elastic material itself. This is an example of an existing product where the silicone used to make the liner has itself been altered by the addition of small additives. The liner does feature a simple, common laminar composite design but it is not the focus of the patent. See EP patent No. 1263358 B1.
Bonded Polymer Laminate Composites
Ossur also makes use of a laminated composite of silicones of varying durometer values. “A stiffer outer layer of DermoSil® silicone provides outstanding stability, while a softer DermoGel® inner layer nurtures the skin and provides shock absorption and comfort.” These liners are currently commercially available. http://www.ossur.com/pages/13399# double-durometer
Advanced Material Configuration Composite Liners
Another patent by Ossur, U.S. Pat. No. 8,308,817, discloses a prosthetic liner composite material that does not use hydrogels but that has a closed distal end, an open proximal end, an outer surface, and an opposite inner surface, the liner for use in prosthetic and orthopedic devices and comprising: a frictional layer formed from a hydrophobic elastomer material and located along the entirety of the inner surface of the liner, the frictional layer defining a plurality of apertures located along the inner surface, the material of the frictional layer having skin tackiness properties; a porous polymer foam layer in communication with the inner surface and directly laminated to the frictional layer, the porous polymer foam layer is a three-dimensional woven synthetic material including discrete portions of a moisture-absorbing material, the apertures of the frictional layer permitting a transfer of air from the inner surface to the porous polymer foam layer; and a cushioning base layer adhered to the porous polymer foam layer and having greater rigidity than the porous polymer foam layer, the base layer forms a liquid and vapor impervious outer surface of the liner; wherein the porous polymer foam layer is a continuous layer extending between the proximal and distal ends of the liner with the base layer extending over the length of the porous polymer foam layer, the porous polymer foam layer permitting a transfer of air from the inner surface of the liner through a thickness of the porous polymer foam layer and out from the proximal end of the liner, wherein the base layer is close-ended at the distal end of the liner, and defines a close-ended conical shape, and wherein the base layer conforms to the shape of the porous polymer foam layer, wherein the base layer is laminated onto the porous polymer foam layer, thereby the frictional layer, the porous polymer foam layer, and the base layer forming a tri-layered laminate structure.
U.S. Pat. No. 6,974,484 discloses a system for removing perspiration from a residual limb inserted in a prosthesis comprising: a nonporous prosthesis socket; a porous thin sheath adjacent the socket; a nonporous liner adjacent the sheath; an osmotic membrane adjacent the liner that purports to allow water vapor to pass from the limb but preventing liquid from passing to the limb; a nonporous seal that prevents air leakage between the residual limb and the socket; and a vacuum source to reduce the pressure in a space between the limb and socket. U.S. Pat. No. 6,974,484 discloses the following materials (but not hydrogels) for the osmotic membrane: Sympatex hydrophylic polyester block copolymer from Sympatex Technologies, One Merrill Industrial Drive, Suite 201, Hampton, N.H. 03842; the Goretex® material from A.W. Gore & Associates, www.gore.com; the Gill 02 Fabric from Gill North America, 1025 Parkway Industrial Park, Buford, Ga. 30581; and the SealSkinz product from Porvair, Estuary Road, King's Lynn, Norfolk, PE30 2HS, United Kingdom. U.S. Pat. No. 6,974,484 claims to use a negative pressure vacuum applied to the socket to provide the pressure gradient needed to drive moisture dissipation away from the limb through and/or around a nonporous liner adjacent the sheath and an osmotic membrane adjacent the liner. However, it is unlikely to work as described. The mechanism it proposes is to use the vacuum to create negative pressure to vaporize the water on the skin, and to draw out the vapor through the osmotic membrane. The pores are of the size such that individual gaseous molecules of water may pass through them but water droplets, being larger in diameter than the pore cannot. This approach is infeasible since the pressure required to vaporize any moisture on the skin would require the skin to be subjected directly to an elevated vacuum thereby weakening the skin and making it vulnerable to skin irritation.
Alternative Materials and Methods
Every interface between man and machine is fundamentally an interface between foreign materials and native cells. Managing this human interface then becomes an exercise in understanding how to best interface with cells. The language of cells is biological, molecular, and microscopic.
It is true, that many interfaces have been successful though they have been designed with a focus on bulk properties only, such as cloth interfaces on healthy tissues. But many times there is a disparity between cellular environment the assistive technology can provide and the needs of the cells. Such an example is the micro climate in the soft tissue interface between the residual limb of an amputee, and the artificial materials used to form the prosthesis.
However a focus on the cellular interface is preferable for developing a device for use on the outer surface of the body as well.
The Material Properties of Interest
1) Young's modulus/elasticity
2) Moisture Permeability
2) How are Materials Tested?
Major findings from: “Testing of elastomeric liners used in limb prosthetics: Classification of 15 products by mechanical performance” Joan E. Sanders, PhD; Brian S. Nicholson, BS; Santosh G. Zachariah, PhD; Damon V. Cassisi, BSME; Ari Karchin, MSE; John R. Fergason, CPO, Departments of Bioengineering and Rehabilitation Medicine, University of Washington, Seattle, Wash.
A number of different elastomeric liner products are available, and manufacturers and users claim they vary in performance. One would then expect that their material properties differ. However, only two reports comparing elastomeric liner material properties have been published:
1. Covey S J, Muonio J, Street G M. Flow constraint and loading rate effects on prosthetic liner material and human tissue mechanical response. J Prosthet Ortho. 2000; 12:15-32.
2. Emrich R, Slater K. Comparative analysis of below-knee prosthetic socket liner materials. J Med Eng Technol. 1998; 22:94-98.
TABLE 1Liners tested. Thickness values reflect those of 10 samples used for compression testing.Table 1.Liners tested. Thickness values reflect those of 10 samples used for compression testing.Mean Thickness (SD)Company (Location)ProductMaterial*(mm)ALPS: St, Petersberg. FloridaEasyLiner ELDT 32-3Silicone gel w/fabric backing4.49 (0.03)EasyLiner ELDT 32-6Silicone gel w/fabric backing5.60 (0.00)EasyLiner SuperSilicone gel6.12 (0.05)Stretch ELPX32Clearpro SSA44Silicone elastomer2.06 (0.05)Engineered Silicone Products: ParsippanyAEGISSilicone elastomer2.19 (0.06)New JerseyAEGIS ZSilicone elastomer w/fabric backing5.10 (0.00)Fillauer. Inc.: Chattanooga. TennesseeSilicone Suspension LinerSilicone elastomer2.01 (0.03)Ohio Willow Wood: Mt. Sterling. OhioAlpha LinerSilicone gel w/fabric backing9.42 (0.09) (front)Ossur USA, Inc.: Columbia, MarylandDERMO Liner-9Gel silicone† w/fabric backing9.29 (0.19)DERMO Liner-6Gel silicone† w/fabric backing5.81 (0.15)Iceross Two ColorSilicone elastomer (two layers)2.27 (0.05)Iceross Comfort, UniformSilicone elastomer w/fabric backing5.89 (0.09)Iceross ClearSilicone elastomer3.36 (0.10)Silipos: New York. New YorkSiloLinerSilicone gel w/fabric backing5.21 (0.11)TEC Interface Systems:Pro 18Urethane6.29 (0.03)Waite Palk. Minnesota*Definition as a silicone gel or silicone elastomer was based on statements in the manufacturers' product literature.†“Gel silicone” is a terms used by this manufacturer. Content and structure are not described to product literature.Table taken from Sanders “Testing of elastomeric liners used in limb prosthetics: Classification of 15 products by mechanical performance”
The difference between silicone elastomers and silicone gels is their cross-linking and fluid retention. Silicone elastomers are extensively crosslinked and contain little free polydimethylsiloxane (PDMS) fluid. Silicone gels have lightly cross-linked polysiloxane networks, swollen with PDMS fluid. Since the PDMS fluid is not chemically bound to the network in silicone gels, fluid can bleed out of the gels.
Thus no conclusions can be drawn about durability. Durability was, however, a topic of previous investigations. Second, testing was conducted under interface loading conditions reflecting those measured at a number of interface locations during walking but not at the patellar-tendon. Thus stresses applied during testing here were lower than patellar-tendon stresses or those experienced during running. High activity, such as running, could induce sweating that could further alter mechanical response.
Several of the liner products had fabric backings on their external surfaces in contact with the socket. The results here showed that the backings' effects on liner tensile stiffness were minimal.
Major Findings from: “Moisture Permeability of the Total Bearing Socket with a Silicone Liner: is it Superior to the Patella-Tendon Bearing Sock”:
Shows no large appreciable differences between liner materials.
But they note that silicone liner is not superior to the PTB socket with regard to moisture permeability, and that it is necessary to develop a new prosthetic socket that allows more heat release and the drainage of sweat. (Hachisuka, 2001)
The thermal conductivity of prosthetic sockets and liners (2007). Hachiuska, K., Matsushima, Y., Ohmine, S., Shinkoda, K., “Moisture permeability of the total surface bearing prosthetic socket with a silicon liner: Is it superior to the patella-tendon bearing prosthetic socket?” Arch. Physic. Med. Rehabil., 82 (2001) 1286-1289.
ThermalconductivityThicknessProductFabric cover(W/m · °K)(mm)descriptionLinerPelite1N0.0854.2Closed cell foamSyncor, Durasleeve2Y0.0853.5Closed cell foamBocklite3N0.0916.0Closed cell foamOWW, Alpha Original3Y0.1143.0Mineral oil gelOWW, Alpha Max2Y0.1286.0Mineral oil gelOWW, P-pod2Y0.1433.0Mineral oil gelOWW, Alpha Spirit2Y0.1556.0Mineral oil gelALPS, EZLiner HP Fabric2Y0.1646.0Silver in gelCentri, Cushion Liner1N0.1643.0ThermoplasticelastomerEuro International, Contex-Gel Streifeneder1Y0.1666.0Polymer gelFreedom Innovations, Evolution SP5N0.1813.0Platinum curedsiliconeALPS, VIVA Sleeve2Y0.1826.0GelMedipro, RELAX2Y0.1826.0Silicone withUmbrellan ®Silipos, Explorer Gel Liner2Y0.1846.0Mineral oil gelESP, Aegis Streamline2Y0.1876.0Pure siliconeESP, Aegis Streamline2N0.1896.0Pure siliconeMedipro Sensitive2Y0.1946.0SiliconeALPS, VIVA Sleeve HP Fabric2Y0.2026.0GelOssur, IceRoss Dermo Seal-in6Y0.2056.0Dermogel ®Euro International, Silicone First Class Liner4Y0.2126.0SiliconeESP, Aegis Ultimate2Y0.2256.0Pure siliconeOttobock, Silicone Liner3Y0.2283.0Silicone gelOssur, IceRoss Comfort Plus Sensil Gel6Y0.2666.0Soft Sensil ®silicone gelSocketCarbon fibre lay-up0.1484.2Thermoplastic0.1504.6OWW, Ohio Willow Wood;ESP, Engineered Silicone Products. Suppliers:1Fillauer, Chattanooga, TN;2Southern Prosthetic Supply, Paso Robles, CA;3Otto Bock, Minneapolis, MN;4Euro International, Tampa, FL;5Freedom Innovations, Irvine, CA;6Ossur, Aliso Viejo, CA.
New hydrogels have recently been developed having increased durability. Previously, swollen hydrogels where weak, crumbling and fracturing under pressure. New types of hydrogels known as dual network hydrogels have increases strength and bridge the gap between traditional rubbers and traditional hydrogels. According to the present disclosure, dual network hydrogels preferably may be used in composites to improve thermal conductivity as well as moisture permeability in various applications including prosthetic socket liners.
FIG. 1 shows the fracture energy of various materials vs their modulus: conventional hydrogels (open square); DN hydrogels (filled star); elastomers (half-filled square); ceramic glasses (filled circle); plastics (diamond); metals (filled triangle). It should be noted that the toughness values for the DN hydrogels have been corrected, because the original references used incorrect formulae for calculating the fracture energy from trouser tear test results.
FIG. 2 illustrates the preparation process and structure of nanocomposite gels: a) clay particles are dispersed in monomer solution; b) initiator is added (which may preferentially adsorb on clay surface); c) polymerization of monomer occurs and polymer chains adsorb on the clay surface; d) final gel structure (as-prepared condition).
Note on Porosity:
Several references herein to “porosity,” “porous,” “pores,” etc. A brief explanation and clarification is needed. Polymeric elastomer materials can generally be thought of as large sponges of entangled polymer chains soaked in a pool of liquid. In the case of silicone, silicone chains are entangled and swollen with silicone oil. This makes them impermeable to water. No water vapor of water liquid can pass through that material. In a molecular level there are free spaces between the chains and oil can flow and pass through. This flow can be measured and a “NET EFFECTIVE POROSITY” can be calculated. This does not mean that there are holes in the silicone, but rather that a pore equivalent behavior can be observed in certain conditions. Hydrogels can similarly be modeled as a swollen network of polymer chains. The chain network is swollen with water. A similar “NET EFFECTIVE POROSITY” can be calculated which described the viscous flow of water through the network, though no actual direct paths, or “holes”, or “pores” exist. This fictional pore is what allows water to pass through the hydrogel, and is on the order of the diameter of a molecule of water or about 2 angstroms. This is in contrast to the “POROUS ELASTOMER” layers that have large holes on the order of microns or millimeters. This is also in contrast to expanded Teflon sheets such as Gortex which have pores that can be observed under magnification. Gortex Pores are true holes and allow vapor and gas through, but are not the correct diameter for passage of liquid water. Hydrogels do not allow gas to flow through them, only liquid water via viscous flow.